In this Fast Track application, we propose developing the novel imaging technology and molecular probe that will enable the non-invasive detection and quantification of HER2 overexpression in breast cancers. The approach we are promoting is to label Herceptin, a therapeutic mono-clonal antibody, with an infraredabsorbing dye, and then to detect the presence of the anitbody/dye conjugate with thermoacoustic computed tomography (TCT) using multi-spectral infrared stimulation. In Phase I we will validate a muIti-spectral decomposition technique for quantifying the presence of an infrared-absorbing dye in the presence of varying amounts of oxygenated and deoxygenated blood. We will also quantify the minimum detectable concentration of dye within a 1-ml volume through a series of tissue-mimicking phantom studies. In Phase II we will collaborate with researchers at Indiana University Medical Center to develop the molecular probe and quantify its binding to HER2 expressing cells. We will also modify our imaging instrumentation to allow us to acquire ultrasound and thermoacoustic CT images, which will be in perfect spatjal registration. Mice will be used to validate our approach in several tumor models with the data collected and equipment developed used to build the foundation for transferring the technology to humans. Our specific Phase I aims: 1. Design, construct and test a TCT imaging system that incorporates a spherically focused, semi-circular detector array. 2. Validate a multi-spectral decomposition technique using this new TCT imaging system for quantifying the concentration of IRIS Green One dye in the presence of oxy- and deoxy-hemoglobin. 3. Determine the minimum detectable concentration of IRIS Green One dye that can be detected within a 1.2-mm diameter tube in a breast-tissue mimicking phantom.